Gallium-nitride-based compound semiconductor such as GaN, InGaN, and GaAlN is drawing attention as material for fabricating blue light emitting diodes (LEDs) and blue laser diodes (LDs) . This kind of compound semiconductor is capable. of emitting blue light of sufficient intensity hardly realized so far.
A blue light emitting element employing the gallium-nitride-based compound semiconductor is disclosed in, for example, Japanese Unexamined Patent Publication No. 4-321280. FIG. 7 shows the basic structure of a blue light emitting element 2 according to a prior art. On a sapphire substrate 200, a buffer layer 201 is formed. On the buffer layer 201, an n-type GaN semiconductor layer 202 and a p-type GaN semiconductor layer 203 are formed. Between the layers 202 and 203, there is a depletion layer to which carriers are injected to emit light.
The blue light emitting element is manufactured by growing crystals on a sapphire substrate according to a CVD method and by forming gallium nitride semiconductor layers on the substrate. The substrate is properly cut into chips. Each chip is connected to a wire frame, and wiring is made to complete a device.
A natural cooling process in an inert gas is disclosed in Japanese Unexamined Patent Publication No. 8-125222. To replace an atmospheric gas at room temperature with an inert gas, the disclosure vacuums a reactive tube under a high temperature. This high temperature may grow a substrate. When vacuuming the reactive tube, the grown crystals may evaporate. As a result, no grown crystals may be left, or the crystallized film may be thinned.
In the gallium-nitride-based blue light emitting element of the prior art, impurities in the semiconductor layers are not sufficiently activated. Accordingly, the prior art needs an after-treatment of thermal annealing.
The thermal annealing increases the number of processes and processing time. Since the gallium nitride semiconductor is exposed to a high temperature of 600.degree. C. or over for a long time, nitrogen may escape from crystals and deteriorate surface homology. This results in changing semiconductor properties and deteriorating blue light emitting efficiency and yield.